Decoration or coating of products with texts and images is commonly made in a wide variety of industries. However, decoration is mostly made in two dimensions, i.e. a two dimensional decoration such as a two dimensional image is applied on a flat surface of a product. Three dimensional decoration on three dimensionally distributed surfaces of products is yet not so common.
In industry, the today most commonly used methods for decoration in three dimensions of medium sized products such as consumer electronics or toys with texts and images are indirect in that a decoration is first printed on a film, whereafter the film is applied on a product so that the decoration adhers to the product. These methods have some disadvantages though, of being complex, slow and expensive.
Some methods for printing a decoration directly on a non-planar, three dimensionally distributed surface, without first printing the decoration onto a film, have been developed.
In U.S. Pat. No. 5,831,641 is described a method for three dimensional printing with inkjet. The idea of this method is to print on a three dimensional surface of an object by using a positioning apparatus which functions to automatically maintain the surface of the object within a plan substantially parallell to and slightly spaced apart from the place within which the inkjet nozzles of the ink-jet plotter reside. This is a straightforward printing method, but it requires advanced positioning mechanics if the shape of the surface is complicated.
U.S. 2001/0019340 A1 describes another method for three dimensional printing with inkjet. In this method, the surface of a printing object is divided into a plurality of target areas, each of which is then approximated by a two dimensional projective plane. An inkjet printhead then prints a projected part image on each target area while moving in parallell with the projective planes. This method decreases the need for positioning mechanics, but introduces the problem with image deterioration owing to the inclination of the object surface in relation to the projective planes and the printhead.
In GB 2351682 A1 there is described an apparatus and a method of spraying a coating, typically a paint, on a selected surface region of an insulating or poorly conducting substrate, such as the base of a bottle, vessel or other receptacle. Sprayed coating material is attracted to the selected surface region by an electrostatic field, which is established by an electrode means and passes through the selected surface region. A conducting member in the form of a plate is disposed in the electrostatic field adjacent the selected surface region and arranged to mask the surface region of the substrate which is bordering the selected surface region. This method has the advantage over other masking arrangements that a sharp boundary between coated and uncoated areas is formed. However, the described method is suited for spraying a paint, not for printing texts and images.
A today conventional inkjet printing system for printing on a two dimensional surface is described in U.S. Pat. No. 4,695,848. According to this document, a drop generator generates ink droplets, which are first charged and then deflected by means of a pair of electrodes positioned on either side of the path followed by the droplets. The deflection is used in order to cover e.g. a whole pixel of an image or character without moving the printhead, but also in order to switch between printing and not printing. Droplets which are not supposed to hit the printing surface are deflected and collected by a gutter.